Phenylketonuria (PKU) is one of the most common genetic disorders affecting children in North America and Europe, with an incidence of about 1 in 10,000 births. Mutations in the enzyme phenylalanine hydroxylase (PAH) are the major cause of PKU. PAH deficiency results in elevated serum phenylalanine (Phe) levels, leading to abnormal brain development and mental retardation. PKU's frequency, coupled with the ability of early dietary modification to greatly reduce the severe neurological problems associated with the disorder, has led to neonatal testing programs in all states. Unfortunately, there is a widespread impression that dietary control effects a "cure" for this disease. In reality, strict adherence to the diet is exceedingly difficult and lapses can cause serious long-term neuro-development sequelae, particularly in offspring of PAH-deficient women. Given the well-understood single gene defect, gene therapy for PKU is both technically feasible and therapeutically desirable. Recent important advances in gene therapy have come from the use of AAV-based vectors to provide effective, stable, and safe correction of metabolic. It is hypothesized that AAV vectors will be useful in PKU. To this hypothesis, the ability of AAV vectors to: 1) Provide effective rescue of defective Phe metabolism in the Pah/enu2 mouse will be examined. Serum Phe levels will be related to the number of vector genomes and transfected cells as well as to the degree of reduction of PKU symptoms. 2) Give safe PAH delivery to liver with non germ-line transmission of vector sequences in either adult and neonatal Pah/enu2 mice will be tested. The potential for liver toxicity and immune responses to vector will also be examined. 3) Optimize PAH expression and hepatocyte delivery by using alternative control elements and tissue-specific targeting. These experiments provide a rational approach to examining the potential of AAV-driven gene therapy to truly cure PKU.